The one-time pad cipher is one common key cryptosystem with which a key is shared between the transmission side and the reception side. With the one-time pad cipher, encryption is carried out using a cipher key of the same amount (the same bit count) as that of communication data. With the one-time pad cipher, a cipher key is used only once and will not be reused.
A typical example of the one-time pad cipher includes a Vernam cipher, with which bitwise exclusive OR or the like of communication data and a cipher key is calculated and the calculated result is treated as encrypted communication data.
The block cipher is one common key cryptosystem, as with the one-time pad cipher. With the block cipher, data is divided into units (usually having a fixed length) called blocks, and each block is encrypted using a cipher key. Normally, with the block cipher, a plurality of blocks are encrypted by the same cipher key.
Examples of the block cipher include Camellia (registered trademark), AES (Advanced Encryption Standard), and the like.
Cipher communication by means of a one-time pad cipher requires a cipher key of a large amount because it consumes the same amount of the cipher key as that of the communication data. With the one-time pad cipher, once the cipher key is exhausted, cipher communication can no longer be performed.
The one-time pad cipher is, however, undecryptable and is much safer than the block cipher.
Patent literature 1 includes a description on the choice of a cryptosystem to apply between the one-time pad cipher and the block cipher depending on the significance of the communication data to be encrypted. This reduces the consumption of the cipher key in the one-time pad cipher.
Patent literature 2 includes a description on how the accumulated amount of the cipher key of each terminal device is monitored so that a cipher key is generated with a higher priority for a terminal device having a small accumulation amount. This is aimed at prevention of the exhaustion of the cipher key in a particular terminal device.